1. Field of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a micromachine package.
2. Description of the Related Art
Micromachine sensing elements (hereinafter micromachine elements) are well known. A micromachine element typically includes a miniature moveable structure, such as a bridge, cantilevered beam, suspended mass, membrane or capacitive element, which is supported over a cavity formed in a silicon wafer. Since the operation of the micromachine element depends upon the moveability of the miniature moveable structure, it is critical that the package, which includes the micromachine element, does not contact the miniature moveable structure in any manner.
FIG. 1 is a cross-sectional view of a structure 8 during the formation of a plurality of micromachine packages in accordance with the prior art. As shown in FIG. 1, a silicon wafer 10 included a plurality of micromachine chips 12. Micromachine chips 12 included micromachine areas 14 on a front surface 10F of wafer 10. Micromachine areas 14 included the miniature moveable structure of the micromachine element. Micromachine chips 12 further included bond pads 16 on front surface 10F of wafer 10. Bond pads 16 were connected to the internal circuitry of micromachine chips 12.
Micromachine chips 12 were often integrally connected together in an array. Each of micromachine chips 12 in the array was delineated by a singulation street 20, which was located between adjacent micromachine chips 12.
A lid 30 was positioned above wafer 10. Lid 30 included a plurality of caps 42 integrally connected to one another. Each cap 42 included a micromachine cavity 32. Each micromachine cavity 32 was positioned over a corresponding micromachine area 14. Generally, micromachine cavities 32 were wider than micromachine areas 14.
Each cap 42 further included a bond pad cavity 34. Each bond pad cavity 34 was positioned over a corresponding set of bond pads 16 on a micromachine chip 12. Generally, bond pad cavities 34 were wider than bond pads 16, and were at least as deep as bond pads 16 were tall.
FIG. 2A is a cross-sectional view of structure 8 of FIG. 1 at a further stage in fabrication in accordance with the prior art. As shown in FIG. 2A, lid 30 was attached to wafer 10. Micromachine cavities 32 were positioned above corresponding micromachine areas 14. Further, bond pad cavities 34 were positioned above corresponding sets of bond pads 16.
FIG. 2B is a cross-sectional view of structure 8 of FIG. 2A at a further stage of fabrication in accordance with the prior art. Referring to FIG. 2B, a series of shallow cuts were made to remove a portion of each cap 42 to expose bond pads 16. Micromachine chips 12 were electrically tested by connecting test probes to bond pads 16. If testing of a micromachine chip 12 indicated that the micromachine chip 12 was defective, the micromachine chip 12 and/or corresponding cap 42 was marked. For example, micromachine chip 12A was marked as being defective. Wafer 10 was then singulated along singulation streets 20. Micromachine chips 12 which were marked as defective were discarded.
Disadvantageously, a cap 42 was attached to a micromachine chip 12 even if the micromachine chip 12 was defective. The cap 42 and defective micromachine chip 12 were discarded. However, since a cap 42 was attached to the defective micromachine chip 12, the cost associated with the defective micromachine chip 12 was increased compared to the cost associated with the defective micromachine chip 12 alone. This increased the cost of fabricating each batch of micromachine packages. This, in turn, increased the average total cost of fabricating each individual micromachine package, which passed testing.
After singulation of wafer 10, each good micromachine chip 12 with cap 42 was further packaged. FIG. 3 is a cross-sectional view of a single micromachine package 40 in accordance with the prior art. As shown in FIG. 3, micromachine chip 12 and cap 42 were attached to a substrate 52. Bond pads 16 were electrically connected to traces 44 by bond wires 46. To prevent accumulation of static charge on cap 42, which would render micromachine chip 12 inoperable, cap 42 was electrically connected to a ground trace 48 by a bond wire 50. Ground trace 48 was grounded during use. Although effective at prevent accumulation of static charge on cap 42, grounding cap 42 by electrically connecting cap 42 to ground through bond wire 50 and ground trace 48 was relatively labor intensive and complex which increased the cost of fabricating package 40.
In accordance with the present invention, a micromachine package includes a micromachine chip mounted as a flip chip to a substrate. The micromachine chip includes a micromachine area and bond pads formed on a front surface of the micromachine chip. The substrate includes traces formed on an upper surface of the substrate. The bond pads are coupled to the traces by bumps, e.g., formed of solder.
Advantageously, the micromachine package is sealed with a package body formed from a cured limited flow liquid encapsulant to protect the micromachine area from the ambient environment. More particularly, the micromachine chip, the package body and the substrate define a cavity, and the micromachine area is located within the cavity.
Recall that in the prior art, the cap was mounted over the micromachine area. Advantageously, by mounting the micromachine chip as a flip chip to the substrate in accordance with the present invention, the requirement for the cap utilized in the prior art is eliminated. By eliminating the cap, the materials and labor associated with manufacturing and installing the cap are also eliminated. Further, since the requirement for the cap is eliminated, the requirement for grounding the cap is also eliminated.
In one embodiment, the limited flow liquid encapsulant is applied in an edge fill configuration for situations requiring a thinner package. In an edge fill configuration, the limited flow liquid encapsulant contacts sides of the micromachine chip.
In another embodiment, a partial overfill configuration is used for those situations having less restriction on total component height. In a partial overfill configuration, the limited flow liquid encapsulant contacts sides of the micromachine chip and extends over at least a portion of a back surface of the micromachine chip.
In yet another embodiment, a full overfill configuration is provided for those situations having little or no restriction on total component height. In a full overfill configuration, the limited flow liquid encapsulant contacts sides of the micromachine chip and extends over and covers the entire back surface of the micromachine chip.
External connectivity configurations, such as a ball grid array (BGA), leadless chip carrier (LCC), or land grid array (LGA) configurations, are provided which are contemplated to be implemented in combination or separate from the embodiments previously described.
Also in accordance with the present invention, a method of fabricating a flip chip micromachine package includes attaching a micromachine chip as a flip chip to a substrate. The micromachine chip has a micromachine area on a front surface of the micromachine chip. The method further includes dispensing a limited flow material, e.g., liquid encapsulant, around the micromachine chip. The limited flow material is cured to form a package body. The micromachine chip, the package body, and the substrate define a cavity, the micromachine area being located within the cavity.
Advantageously, only a micromachine chip which has been tested and found to operate correctly is attached to the substrate. In this manner, waste of the substrate is avoided and labor associated with attaching a defective micromachine chip thereto is saved.
Recall that in the prior art, a single lid was attached to a plurality of micromachine chips while still in wafer form. Thus, in the prior art, a cap was attached to each micromachine chip even if the micromachine chip was defective. Accordingly, caps and labor associated with attaching caps to the defective micromachine chips were wasted in the prior art. Since this waste of the prior art is eliminated, the micromachine package in accordance with the present invention is less expensive to manufacture than a micromachine package of the prior art.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.